Distributed fiber temperature monitoring for power cables and utility tunnels

Project background: from isolated alarms to continuous risk assessment

Distributed fiber temperature monitoring for power cables and utility tunnels addresses the long-term safety and condition management of high-voltage cables, intermediate joints, terminations, cable trays, cable trenches and urban utility tunnels. Such assets commonly span multiple spaces, process sections or asset identifiers; risks can move along routes, equipment boundaries and structural weaknesses rather than appearing at predetermined measuring points. Key concerns include combined risks from increased joint resistance, insulation aging, local overload, external fire, standing water and ventilation changes. Point sensors, manual inspections and video provide local evidence but may not capture the complete development of an anomaly from onset to response.

The solution uses DTS, centimeter-scale DTS and high-temperature sensing cables, placing sensing fiber along the paths that need observation to create continuous, localized and replayable records. Its objective is to form continuous temperature profiles, locate abnormal heating, distinguish steady load heating from rapid thermal faults and map alarms to cable identifiers and maintenance sections, rather than add isolated equipment. Start design with an asset-risk inventory and verifiable monitoring objectives, then specify range, spatial resolution, sampling, sensing-cable construction, software interfaces and alarms.

Risk progression and monitoring boundaries

For high-voltage cables, intermediate joints, terminations, cable trays, cable trenches and urban utility tunnels, distinguish initiating factors, early signs, developing conditions and consequences. Risks including combined risks from increased joint resistance, insulation aging, local overload, external fire, standing water and ventilation changes may occur independently or arise from combined load, environmental, construction and maintenance effects. Capture measurable changes in temperature, acoustics, vibration, strain or local condition and associate them with specific assets and distances.

Sensing principles and technology selection

A DTS interrogator launches short laser pulses into sensing fiber and calculates temperature along the fiber from returned Raman backscatter. Engineering value lies in continuous temperature profiles, accurate distance to anomalies, heating rates and heat-propagation direction rather than isolated readings. The sensing cable acts as both measurement medium and a long temperature array, without closely spaced power supplies along its route. This suits strong electromagnetic fields, damp or dusty locations, hazardous areas and sites with difficult maintenance access, subject to the required equipment and installation approvals.

The recommended combination is DTS, centimeter-scale DTS and high-temperature sensing cables. Check monitoring distance, spatial resolution, localization accuracy, channel count, dynamic range, sampling cycle, operating temperature and communications together. Specifications should support the objective to form continuous temperature profiles, locate abnormal heating, distinguish steady load heating from rapid thermal faults and map alarms to cable identifiers and maintenance sections, rather than be compared individually. For long routes, prioritize signal-to-noise ratio, reliable localization and far-end stability; for localized high-risk areas, assess coupling, effective sensing length and response time.

System architecture: sensing, interrogation, software and response

The system comprises sensing cable or fiber sensors, splicing and protection units, interrogation equipment for DTS, centimeter-scale DTS and high-temperature sensing cables, edge processing, application software and supervisory interfaces. Returned signals undergo quality checks, distance calibration and parameter calculation before mapping to asset records. Outputs include live curves, time-space plots, event lists, risk zones, historical trends and reports, rather than an unexplained waveform.

Connect to fire alarms, SCADA, BMS or integrated maintenance systems with zoned relay outputs, Modbus TCP and historical trends. Define the data dictionary, time synchronization, alarm acknowledgment, offline buffering and recovery upload. For important projects, use tiered retention of raw or feature data to support incident reviews, algorithm improvement and threshold changes.

Sensing cable and field installation

Recommended installation: route sensing cable by circuit, tray level and fire compartment, increase effective coupling at dense joints and bends and place interrogators in station or fire-control rooms. Installation quality directly affects results; the same interrogator can respond differently with different mounting, jacket materials and coupling. Design documents should identify start/end distances, slack loops, splice boxes, zone boundaries, fastening intervals, bend radius, pulling tension and protection.

Data analysis and graded alarms

The alarm model combines absolute temperature, rate of rise, differences between adjacent sections, differences between similar assets and duration. Establish normal baselines for season, load, ventilation and process state before detecting deviations. Centimeter-scale DTS suits small heat sources and dense localization, while standard and long-range DTS suit continuous routes. Select by asset size, cable coupling and permitted response time, rather than ranking systems solely by spatial resolution.

Initial application criteria: combine absolute temperature, heating rate, temperature difference and duration, with baselines for work schedules, seasons and load levels. Use advisory, early-warning, alarm and emergency levels, each with defined confirmation times, verification and actions. Calibrate thresholds against actual conditions during trial operation and record every revision, reason and effective date. Display position, asset name, measured value, baseline deviation, duration and supporting evidence together.

Commissioning, acceptance and performance verification

Acceptance should use zoned heating, controlled movement of heat sources and repeated tests under different operating conditions to verify localization error, response time, temperature repeatability and alarm interlocks. Heating a short length next to the interrogator alone is insufficient; sample the far end, joints, bends and positions with unfavorable heat transfer.

Maintenance and sustained effectiveness

Maintenance assessment should cover valid-alarm rates, confirmation time, false-alarm causes, missed-event reviews, closed work orders and detected trends, not just equipment uptime. Continued labeling of real events can improve recognition models and sustain practical performance after commissioning.

Integration with existing systems

Connect to fire alarms, SCADA, BMS or integrated maintenance systems with zoned relay outputs, Modbus TCP and historical trends. Let the fiber platform handle interrogation, curve analysis and original evidence, while the supervisory system manages consolidated displays, duty workflows and work orders. Depending on the project, interfaces may use Modbus TCP, OPC UA, IEC protocols, REST APIs, message queues or dry contacts. Avoid transferring only an unexplained aggregate alarm.

Every alarm on a map, plan or process diagram should link back to original curves and historical trends. Feed dispositions from supervisory systems back into event records, closing the loop between monitoring, confirmation, response, review and threshold improvement.

Benefits, operating conditions and technical boundaries

The solution replaces sampled inspections of high-voltage cables, intermediate joints, terminations, cable trays, cable trenches and urban utility tunnels with continuous observation, retaining traceable early indications of combined risks from increased joint resistance, insulation aging, local overload, external fire, standing water and ventilation changes. Passive fiber, electromagnetic immunity and multiple sensing locations on one fiber can reduce distributed power and maintenance requirements on long routes, at unattended sites or in hazardous environments. Locations and trends help teams narrow inspections, plan condition-based maintenance and prioritize risk.

Equipment selection and detailed design

Begin with five questions: which parts of high-voltage cables, intermediate joints, terminations, cable trays, cable trenches and urban utility tunnels require protection; which indications of combined risks from increased joint resistance, insulation aging, local overload, external fire, standing water and ventilation changes must be detected; what localization, response and sampling performance is needed; what routing, power, communications and cabinet facilities are available; and who confirms alarms and owns the response workflow?

Then specify the combination of DTS, centimeter-scale DTS and high-temperature sensing cables, cable construction, single- or double-ended optical paths, channel redundancy, software licensing and interfaces. For phased projects, reserve fiber, rack space, network addresses and platform capacity. For long-distance or multisite projects, evaluate centralized interrogation, distributed edge nodes and remote maintenance together.

Application-specific validation and limitations

Establish separate heating baselines for joints and cable bodies. Spare loops, coils and cross-level routes near joints alter the relationship between optical and field distance; retain cable IDs, joint IDs and fiber positions on drawings. Under load, compare joints with adjacent cable bodies and circuit load rather than comparing absolute temperatures across unrelated circuits.

Align utility-tunnel zones with fire doors, branches and entrances; operators should locate physical points from alarm positions during handover. Record curve changes after flooding, ventilation switching or cable additions. Where joints cannot be de-energized, choose practicable fixing methods within electrical-work requirements rather than compromising insulation or safety clearance for stronger thermal coupling.

Installation, optical-path calibration and acceptance checklist

Related products and technical resources

Explore the product categories below according to distance, spatial resolution, channels, response and site conditions. Published specifications support preliminary selection; final configuration must account for optical loss, installation and acceptance objectives.